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Learning Objective

Key Points

Knowing the molar mass of an element or compound can help us stoichiometrically balance a reaction equation.

The average molar mass of a mixture of gases is equal to the sum of the molefractions of each gas (xi) multiplied by the molar mass (Mi) of that particular gas: $\bar { M} =\sum _{ i }^{ }{ { x }_{ i }{ M }_{ i } }$ .

a hypothetical gas whose molecules exhibit no interaction and undergo elastic collision with each other and with the walls of the container

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Molar Mass of Gases and Gas Mixtures

Molar mass (M) is equal to the mass of one mole of a particular element or compound; as such, molar masses are expressed in units of grams per mole (g mol–1) and are often referred to as molecular weights.
The molar mass of a particular gas is therefore equal to the mass of a single particle of that gas multiplied by Avogadro's number (6.02 x 1023 ).
For a mixture of gases, the molar mass depends on the molar masses of each of its components and on the fractional abundance of each kind of gas in the mixture.
In this case, the term "average molecular weight" is often used to describe the average molar mass of the gas mixture.

The average molar mass of a mixture of gases is equal to the sum of the mole fractions of each gas, multiplied by their respective molar masses:

$\bar { M} =\sum _{ i }^{ }{ { x }_{ i }{ M }_{ i } }$

The molar volumes of all gases are the same when measured at the same temperature and pressure (22.4 L at STP), but the molar masses of different gases will almost always vary.